Re: When are index scans used over seq scans?

Tom Lane wrote:
> which is almost 80% of the entire runtime.  Which is enormous.
> What are those column datatypes exactly?

                      Table "richard.sessions"
         Column         |            Type             | Modifiers
------------------------+-----------------------------+-----------
[unrelated columns removed]
 starttimetrunc         | timestamp without time zone |
 finishtimetrunc        | timestamp without time zone |
Indexes:
    "rb_us_st_ft_idx" btree (starttimetrunc, finishtimetrunc)
    "rb_us_st_ft_idx2" btree (finishtimetrunc, starttimetrunc)
Check constraints:
    "date_check" CHECK (finishtimetrunc >= starttimetrunc)

             Table "richard.duration"
 Column |            Type             | Modifiers
--------+-----------------------------+-----------
 ts     | timestamp without time zone |

> Perhaps you are incurring a datatype conversion cost?

Not that I can tell.

> It seems more likely that the cpu_operator_cost is underestimated,

As you perdicted, increasing cpu_operator_cost from 0.0025 to 0.025 also
causes the planner to use the index on duration.

> which leads me to question what exactly is happening in those
> comparisons.

Your guess is as good as mine (actually, yours is much better). I can
put together a reproducable test case if you like..

--
Richard van den Berg, CISSP
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